The present invention relates to an improved optical system for copying machines in which an original image is formed only once into a visible image on a light sensitive drum, the visible image is used as a secondary original which is focused on another light sensitive drum, the focused image is transferred to a paper or a film after processes such as development, and a number of copies can be produced at a high speed.
Various optical systems in which a number of copies can be obtained with one scanning operation of the original have been proposed. There has been proposed one system in which two light sensitive drums are used. This system will be described briefly.
FIG. 1 is a schematic view of a prior art copying optical system of this type. An original O is illuminated by an illumination device F.sub.1 and is projected onto a first drum D.sub.1 by an ordinary copying optical system including a mirror M and a lens L.sub.1. The first drum D.sub.1 is charged in advance. When projected, the image is formed as an electrostatic latent image on the first drum D.sub.1. Then, the image is developed as a visible image. Until a required number of copies are obtained, the visible image is used as a secondary original. The thus formed visible image is illuminated by a second illumination device F.sub.2 disposed close to the first drum D.sub.1 and is focused and projected on a second drum D.sub.2 through a lens L.sub.2 of a second optical system as a secondary original image. Also, the second drum D.sub.2 is charged in advance and the image thereon is developed and transferred onto a paper or a film thereby obtaining copies. The path from the first drum D.sub.1 to the second drum D.sub.2 is hereinafter referred to as a second optical path.
As described above, in the prior art two-drum type copying machine, separate lenses, convergent fibers and mirrors are used in the first and second optical paths, respectively. In this prior art optical system there are disadvantages in that two generally expensive lenses are required, the number of mechanical parts which are used to mount the lenses is great and the machining processes therefore are accordingly numerous, and the adjustment of the drums and optical parts is complicated thereby resulting in a high manufacturing cost.